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Bergslagen District Iron - Grangesberg, Idkerberget, Haksberg, Strassa, Stripa, Riddarhyttan |
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Sweden |
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Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Bergslagen District in southern Sweden extends over an arcuate area of approximately 200 x 50 km some 150 km west of Stockholm and hosts a range of ore deposits that have been exploited over a long historical period. These include iron as well as base (Cu, Pb, Zn) and precious (Au, Ag) metals. Ths record concentrates on the iron deposits, while a separate record covers the base metal mineralisation.
In 1861 there were 511 operating iron mines in the district producing 425 000 tonnes of ore for the year, as well as 23 base metal mines with an output of 130 000 tonnes of ore. by 1911 there were 242 iron mines for 2.15 Mt of ore and 26 base metal mines for 278 000 tonnes of ore. In 1973 22 iron mines contributed 8.80 Mt of ore and 9 base metal mines yielded 1.22 Mt of sulphide ore.
The country rocks of the district comprise a Palaeoproterozoic (~1900 Ma) metamorphosed volcano-sedimentary succession composed mainly of submarine rhyolitic volcanic, sub-volcanic and volcaniclastics with subordinate mafic volcanics, chemical, epiclastic and organogenic (carbonate) sediments spread through the pile. See the Bergslagen Base Metals record for more detail of the geological and tectonic setting of the district.
The majority of the ores have been interpreted as being skarn altered carbonates interlayered with volcanogenic ash-siltstones, but apparently related to later sub-volcanic intrusives. These hosts were subsequently overlain by argilites, greywackes, quartzites and conglomerates and intruded by early-orogenic ultramafics to granitoids, late-orogenic granites and post-orogenic plutons and even later dolerites. The principal orogenesis is associated with the 1800 to 1750 Ma Svecikarelian orogenic event.
The great majority of the numerous mineral occurrences and ore deposits represent iron mineralisation that has been sub-divided into the following groups:
• Banded quartz-hematite (±magnetite) ores - occurring as thin (mm to cm scale) alternating layers of hematite and quartz paralleling bedding and totalling around 20 m in thickness with an ore grade of around 50% Fe. Hematite is locally reduced to magnetite.
• Skarn limestone magnetite ore - occurring as beds or massive to disseminated magnetite replacement of skarn altered carbonates, locally accompanied by manganese and or base metals. Ore grades range from 43 to 62% Fe and 0 to 5% Mn. Several deposits have ore grade base metals.
• Massive apatite-rich magnetite (±hematite) ore - occurring as massive replacements in extrusive volcanic hosts and in part by sub-volcanic intrusives emplaced in the latter and are interpretted to have a 'porphyry type' affiliation. All of the deposits of this type in Central Sweden are confined to a single 40 km long zone. An example of this type is the Grängesberg deposit, the single largest producer in the region responsible for 150 Mt of ore. Grades are generally 56 to 63% Fe with 0.9 to 1.3% P. Phosphate is sometimes of sufficient grade to be mined as a by-product. This style is similar to that at Kiruna much futher north in Norbotten.
• Disseminated apatite-bearing magnetite ore - the apatite-rich magnetite ores commonly grade laterally and stratigraphically into disseminated mineralisation.
In addition to these iron ores and occurrences, base and precious metal deposits are also found in the district (see the Bergslagen Base Metals record for more detail.).
A selection of the more significant iron ore deposits of the Bergslagen District are as follows:
Grängesberg-Risberg - this was the largest and most important of the apatite bearing magnetite deposits in the Bergslagen district. Mining activity on the deposit commenced in the 16th century and continued unabated until closure in 1990. The deposit falls within a narrow 40 km long belt that contains all of the significant magnetite-apatite deposits in central Sweden. To the NNE of Grängesberg are the smaller occurrences of Blötberget, Fredmundberget and Lekomberg. Idkerberget is 20 km to the NNE.
Mineralisation is hosted by coarse metamotphosed acid volcanics, locally termed leptites. These leptites, which form a 2000 m thick succession in the district, vary from potash rich to extreme soda rich types (quartz-keratophyres) with subrodinate intermediate volcanics. Calcareous intercalations are found within the volcanics, varying from magnesia poor to magnesia rich.
The Grangesberg iron ores are found low in the leptite sequence and represent the oldest of a number of ore horizons found within the sequence, each with its own distinctive character and mineralogy. The Grängesberg ores occur in association with intermediate or basic volcanics in a more volcanic rich part of the succession, characterised by an abundance of agglomerates and dykes and the wedging out of lavas away from the mineralisation.
Contacts between ore and hosts vary from sharp to gradational, via intervals of rich to lean magnetite-apatite disseminations, often accompanied by the development of skarn minerals in the host. Dacite dykes cut both ore and wall rocks, but have not been skarn altered. Small apatite rich (up to 40%) masses are found within the iron ores.
The main ores are mainly magnetite, with lesser hematite locally. The gangue is composed of quartz and skarn minerals (dominantly actinolite and hornblende, with lesser biotite, muscovite, chlorite, epidote and garnet. The skarn minerals are best developed in the higher grade ore zones, while massive skarn development accompanies thinning of ore. Disseminated hematite is found in the leptites adjacent to the apatite bearing magnetite ores where they form breccias composed of large and small clasts of leptite.
At Grängesberg the host grey sodic leptites form a narrow belt around 500 m wide enclosed on both sides by older granites, which are grey and salic to the west and grey and intermediate to the east.
The main deposit is lensoid, conformable, around 1000 m in strike length and 20 to 90 m wide, with a SSE plunge and NE-SW strike. At either end the ore wedges out into the leptites via several lenses, while lenses and bands of leptite are found within the ore zone. The average grade is 60% Fe with around 1% P. Extensive zones (several km in strike length) of disseminated hematite is found in red potassium leptite in the footwall of the main ore zones, often exhibiting a breccia structure and intervals of high grade ore. Production from the Export field at Grängesberg in 1973 totalled 4.2 Mt of ore grading 59.4% Fe, 0.89% P, 0.12% Mn. Production from Grängesberg to 1973 totalled 150 Mt. During the mine’s life, more than 180 Mt of ore were produced.
Idkerberget - is around 35 km north of Grängesberg and lies within a 40 km long NE-SW trending isoclinal syncline. The deposit is embraced by strongly gneissic leptites accompanied by concordant beds of amphibolite and hornblende-biotite skarn. The main ore lenses occur mostly within and occupy most of the width of a 60 m thick and 750 m long lense of amphibolite that extends to a depth of at least 900 m. Magnetite, accompanied by hornblende and apatite is the dominant ore mineral, with lesser local hematite, particularly on the ends and outer contacts of the ore. The average grade of the ore is 62% Fe, 0.6 to 0.7% P, 0.02 to 0.03% S. Total production between 1925 and 1974 was 9.3 Mt.
The Idkerberget field extends east through the Skvatterber and Haggruvan mines which are developed within leptite 'xenoliths' within granite masses. Other deposits and swarms of smaller exploited lenses are found in fields several kilometres in length and up to a kilometre wide within a few km of Idkerberget.
The Haksberg field - lies within another isoclinal syncline to the east of Idkerberget where the host leptites are overlain by greywackes and schists, enclosed on both sides by older Svecofennian granites. The sequence has been strongly deformed and metamorphosed. This syncline hosts many parallel ore zones and lenses at different levels in the sequence. Low within the leptite sequence the mineralisation comprises a series of parallel lenticular quartz-banded iron ore lenses, sometimes repeated by isoclinal folding, defining a 17 km belt of deposits and occurrences which includes Haksberg. At the top of the leptites, immediately below the contact with the overlying sediments, there are manganses rich iron ores and sulphide deposits. Near the base of the leptite sequence the ores are more skarn rich and hosted by a more strongly carbonate bearing sequence with associated sodic leptites.
While magnetite predominates in the quartz-banded iron ores at Haksberg, all transitions from magnetite to hematite are found, and the ore exhibits well preserved quartz banding with the quartz bands being broken up. Abundant later chalcopyrite and bornite are found in some of these same lenses. The gangue includes chlorite, epidote, plagioclase, mica and garnet. Ore grades vary from 34 to 55% Fe and 0.03 to 0.1% P from a total production of 15.5 Mt from 1935 to 1975.
Stripa, Strassa, Blanka, Riddarhyttan & Norberg - are part of a 100 km long NE-SW trending belt (some 20 km SE of the belts of apatite bearing iron ores) that accounts for around 30% of the known iron reserves in central Sweden. The leptite-sediment host sequence is again intensely deformed and bounded by older Svecofennian granites.
The belt contains deposits with three types of ore, namely: 1). manganese rich iron ores; 2). quartz-banded ores; and 3). quartz-bearing skarn ores interlayered with carbonate ores.
These deposits are found in both leptites and carbonates in the central carbonate rich section of the sequence and include some of the largest non-apatitic iron deposit in central Sweden.
Many of the iron ores have a significant sulphide component, mainly disseminated pyrite/pyrrhotite and chalcopyrite (+fluorite) with zones of several hundred thousand tonnes which carry up to 1% Cu as at Blanka. Silver & uranium rich ores are also found associated with some of the iron deposits in the belt.
Iron ores are found as both quartz-banded hematite (+magnetite) ores, and as homogeneous to only faintly banded, coarse grained and massive magnetite, both of which are found in a series of lensoid, conformable lenses with containing around 35% Fe, although reserves with grades of 40-55% are also recorded from some of the larger deposits in the belt.
Between 1925 and 1974 production from these groups of mines totalled: Norberg - 22 Mt; Riddarhyttan - 11 Mt; Strassa, Blanka - 19.5 Mt; Stripa - 10 Mt;
The most recent source geological information used to prepare this decription was dated: 2002.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Grip, E., 1978 - Sweden - Extract on the Bergslagen District of Central Sweden: in Bowie, S.H.U., Kvalheim, A. and Haslam, H.W., 1978 Mineral Deposits of Europe, The IMM, London, v.1, pp. 97-138.
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Hallberg, A., Bergman, T., Gonzalez, J., Larsson, D., Morris, G. A., Perdahl, J. A., Ripa, M., Niiranen, T. and Eilu, P., 2012 - Metallogenic areas in Sweden: in Eilu, P., 2012 Mineral deposits and metallogeny of Fennoscandia, Geological Survey of Finland, Special Paper 53, pp. 139-206.
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Tragardh, J., 1991 - Metamorphism of magnesium-altered felsic volcanics rocks from Bergslagen, central Sweden. A transition from Mg-chlorite- to cordierite-rich rocks: in Ore Geology Reviews v.6, pp. 485-497.
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Weihed P, Arndt N, Billstrom K, Duchesne J C, Eilu P, Martinsson O, Papunen H and Lahtinen R 2005 - Precambrian geodynamics and ore formation: The Fennoscandian Shield : in Ore Geology Reviews v27 pp 273-322
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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